The present disclosure relates to a comb-based coherent LIDAR that provides a unique combination of precision, speed and large ambiguity range.
In intrasatellite ranging, and similarly in manufacturing applications, there are three critical parameters: precision/accuracy, ambiguity range and update rate. High precision is particularly important in maintaining the pointing during formation flying of satellites; for example, coherent combining of 1-m sub-apertures to form a synthetic aperture of 100-m diameter requires a relative pointing accuracy of less than (λ/100 m) rad for the sub-aperture on each satellite, which in turn requires distance measurements at the sub-aperture edges with less than λ×(1 m/100 m) accuracy, or a few nanometers at optical wavelengths. The ambiguity range characterizes the measurement range window; longer distances are aliased back to within the ambiguity range. Larger ambiguity range requires less a priori distance knowledge. Finally, fast millisecond-scale update rates are needed for effective feedback.
Many of these requirements push or exceed the capabilities of current ‘stand off’ ranging technology. Generally speaking, laser ranging is the determination of the phase shift on a signal after traversing a given distance. Crudely, shorter-wavelength signals offer greater resolution, and longer wavelength signals offer greater ambiguity range. For instance, the widely used continuous-wave (c.w.) laser interferometer measures the phase of optical wavelengths to achieve sub-nanometer resolution. However, measurements are limited to relative range changes as the ambiguity range equals half the laser wavelength. Alternatively, laser radar (LIDAR) measures distance through pulsed or radio-frequency (rf)-modulated waveforms. For pulsed systems, the time-of-flight is measured
These systems offer large ambiguity ranges but with ˜50-100 μm resolution. Multiwavelength interferometry (MWI) combines measurements at several optical wavelengths, which effectively generates a longer ‘synthetic wavelength’, and therefore a reasonable ambiguity range while maintaining sub-wavelength resolution. However, these systems are vulnerable to systematic errors from spurious reflections, and extending the ambiguity range beyond a millimeter can require slow scanning. Nevertheless, with extensive care in minimizing spurious reflections, the MSTAR system has successfully used MWI for sub-micrometer ranging. Femtosecond optical frequency combs offer an intriguing solution to intrasatellite ranging. From the early work by Minoshima, combs have been incorporated into precision ranging systems using various approaches.
Although frequency combs are normally considered in the frequency domain where they produce a comb of well-defined, narrow linewidth optical frequency lines, these same sources can also be viewed in the time-domain where they produce a train of well-defined, coherent optical pulses. These optical pulses can be very short in duration, i.e. have a large bandwidth, and can be arranged to have a high carrier phase coherence with an underlying optical cw “clock” laser. They very much resemble a coherent RADAR pulse train, except that the carrier frequency is shifted up into the optical region and their bandwidth can be significantly larger. As a consequence, these sources are interesting for high-resolution coherent LIDAR systems.
One challenge of taking full advantage of these high-bandwidth, coherent sources in a coherent LIDAR system lies in effectively detecting the return signal. The optical pulses have very high bandwidth (i.e. THz or greater) and standard direct or heterodyne detection would require an equivalently large bandwidth receiver.